1. Field of the Invention
The present invention relates to a plasma treatment method by which substrates such as semiconductor wafers are etched or sputtered under plasma atmosphere. It also relates to a plasma treatment apparatus for the same.
2. Description of the Related Art
Recently, semiconductor devices are more and more highly integrated and the plasma treatment is therefore asked to have a finer workability in their making course. In order to achieve such a finer workability, the process chamber must be decompressed to a greater extent, plasma density must be kept higher and the treatment must have a higher selectivity. In the case of the conventional plasma treatment methods, however, high frequency voltage becomes higher as output is made larger, and ion energy, therefore, becomes stronger than needed. The semiconductor wafer becomes susceptible to damage, accordingly. Further, the process chamber is kept about 250 mTorr in the case of the conventional methods and when the degree of vacuum in the process chamber is made higher (or the internal pressure in the chamber is made smaller), plasma cannot be kept stable and its density cannot be made high.
When gases are made plasma, the action of ions in the plasma becomes different, depending upon frequencies of high frequency power. In short, ion energy and plasma density can be controlled independently of the other when high frequency power having two different frequencies is applied to process gases. However, ions (loaded particles) easily run from plasma to the wafer at a frequency band, but it becomes difficult for them to run from the plasma sheath to the wafer at another frequency band (or transit frequency zone). The so-called follow-up of ions becomes unstable.
Particularly molecular gases change their dissociation, depending upon various conditions (such as kinds of gas, flow rate, high frequency power applying conditions and internal pressure and temperature in the process chamber), and the follow-up of ions in the plasma sheath changes in response to this changing dissociation. Further, the follow-up of ions at the transit frequency zone also depends upon their volume (or mass). Particularly in the case of molecular gases used in etching and CVD, the dissociation of gas molecules progresses to an extent greater than needed when electron temperature becomes high with a little increase of high frequency power, and the behavior of ions in the plasma sheath changes accordingly. Plasma properties such as ion current density become thus unstable and the plasma treatment becomes uneven, thereby causing the productivity to be lowered.
When the frequency of high frequency power is only made high to increase plasma density, the dissociation of gas molecules progresses to the extent greater than needed. It is therefore desirable that the plasma density is raised not to depend upon whether the frequency is high or low.
An object of the present invention is therefore to provide plasma treatment method and apparatus capable of controlling both of the dissociation of gas molecules and the follow-up of ions and also capable of promoting the incidence of ions onto a substrate to be treated.
Another object of the present invention is to provide plasma treatment method and apparatus capable of raising the plasma density with smaller high frequency power not to damage the substrate to be treated.
According to the present invention, there can be provided a plasma treatment method of plasma-treating a substrate to be treated under decompressed atmosphere comprising exhausting a process chamber; mounting the substrate an a lower electrode; supplying plasma generating gas to the substrate on the lower electrode through an upper electrode; applying high frequency power having a first frequency f1, lower than the lower limit of ion transit frequencies characteristic of process gas, to the lower electrode; and applying high frequency power having a second frequency, higher than the upper limit of ion transit frequencies characteristic of process gas, to the upper electrode, whereby a plasma generates in the process chamber and activated species influence the substrate to be treated.
It is preferable that the first frequency f1 is set lower than 5 MHz, more preferably in a range of 100 kHz-1 MHz. It is also preferable that the second frequency f2 is set higher than 10 MHz, more preferably in a range of 10 MHz-100 MHz.
High frequency power having the frequency lower than the lower limit of ion transit frequencies is applied to the lower electrode. Therefore, the follow-up of ions becomes more excellent and ions can be sore efficiently accelerated with a smaller power. In addition, both of ion and electron currents change more smoothly. Further, the follow-up of ions does not depend upon kinds of ion. The plasma treatment can be thus made more stable even when the degree in the process chamber and the rate of gases mixed change. On the other hand, high frequency power having the frequency higher than the upper limit of ion transit frequencies is applied to the upper electrode. Therefore, ions can be left free from frequencies of their transit frequency zone to thereby enable more stable plasma to be generated.
Ion transit frequency zones of process gases used by the plasma treatment in the process, such as etching, CVD and sputtering, of making semiconductor devices are almost all in the range of 1 MHz-10 MHz.
Impedances including such capacitive components that the impedance relative to high frequency power becomes smaller than several kxcexa9 and that the impedance relative to relatively low frequency power becomes larger than several xcexa9 are arranged in series between the upper electrode and its matching circuit and between them and the ground. Current is thus made easier to flow to raise the plasma density and ion control.